Reduction of the hand representation in the ipsilateral primary motor cortex following unilateral section of the corticospinal tract at cervical level in monkeys
- Eric Schmidlin†1,
- Thierry Wannier†1, 2,
- Jocelyne Bloch3,
- Abderraouf Belhaj-Saif1,
- Alexander F Wyss1 and
- Eric M Rouiller1Email author
© Schmidlin et al; licensee BioMed Central Ltd. 2005
Received: 11 April 2005
Accepted: 31 August 2005
Published: 31 August 2005
After sub-total hemi-section of cervical cord at level C7/C8 in monkeys, the ipsilesional hand exhibited a paralysis for a couple of weeks, followed by incomplete recovery of manual dexterity, reaching a plateau after 40–50 days. Recently, we demonstrated that the level of the plateau was related to the size of the lesion and that progressive plastic changes of the motor map in the contralesional motor cortex, particularly the hand representation, took place following a comparable time course. The goal of the present study was to assess, in three macaque monkeys, whether the hand representation in the ipsilesional primary motor cortex (M1) was also affected by the cervical hemi-section.
Unexpectedly, based on the minor contribution of the ipsilesional hemisphere to the transected corticospinal (CS) tract, a considerable reduction of the hand representation was also observed in the ipsilesional M1. Mapping control experiments ruled out the possibility that changes of motor maps are due to variability of the intracortical microstimulation mapping technique. The extent of the size reduction of the hand area was nearly as large as in the contralesional hemisphere in two of the three monkeys. In the third monkey, it represented a reduction by a factor of half the change observed in the contralesional hemisphere. Although the hand representation was modified in the ipsilesional hemisphere, such changes were not correlated with a contribution of this hemisphere to the incomplete recovery of the manual dexterity for the hand affected by the lesion, as demonstrated by reversible inactivation experiments (in contrast to the contralesional hemisphere). Moreover, despite the size reduction of M1 hand area in the ipsilesional hemisphere, no deficit of manual dexterity for the hand opposite to the cervical hemi-section was detected.
After cervical hemi-section, the ipsilesional motor cortex exhibited substantial reduction of the hand representation, whose extent did not match the small number of axotomized CS neurons. We hypothesized that the paradoxical reduction of hand representation in the ipsilesional hemisphere is secondary to the changes taking place in the contralesional hemisphere, possibly corresponding to postural adjustments and/or re-establishing a balance between the two hemispheres.
Although voluntary dexterous movements of the hand are mainly under control of motor cortical areas in the opposite hemisphere, there is evidence that the ipsilateral motor cortex may also contribute, but to a lesser extent. For instance, the activity of single neurons in the primary (M1), supplementary (SMA), premotor (PM) and cingulate (CMA) motor cortical areas was found to be modulated when monkeys performed movements with the ipsilateral hand. [1–7]. Intracortical microstimulations in the primary motor cortex in monkeys were reported to evoke not only the expected movements of the contralateral digits  but also responses of the ipsilateral fingers . In human subjects, there is increasing evidence that the motor cortex is involved in the control of ipsilateral hand movements (e.g. [10–16]). The possible contribution of the motor cortex in the control of the ipsilateral hand may be important for normal function, although its precise role has not been elucidated yet. Furthermore, it has been anticipated that the ipsilateral motor cortex may be crucial for recovery of motor function of a paretic hand after unilateral brain lesion, such as after stroke (e.g. [17–20]). However, the involvement of the intact hemisphere in the control of the ipsilateral paretic hand remains a matter for debate. [21–23]. Moreover, there is clear evidence, in both monkeys and human subjects, that a significant re-organization of motor maps takes place in the affected hemisphere after unilateral lesion of M1. [24–28], a re-arrangement crucially involved in the functional recovery of the paretic hand .
Regarding lesions of the spinal cord, several studies reported plastic changes of motor maps in the cerebral cortex in both monkeys [29–31] and human subjects [32–37]. In a recent report, we described in detail the changes of motor maps that occurred in the motor cortex contralateral to a unilateral section of the corticospinal (CS) tract at cervical level C7/C8 . Several months after the lesion, the contralesional hemisphere showed a dramatic decrease of the hand representation, compared with before the lesion, ranging from 69 to 97 % depending on the site and extent of the lesion. The progressive changes in hand representation occurred during the 30–40 days post-lesion, in parallel to the functional recovery of the affected hand. Finally, we demonstrated that the re-arranged contralesional motor cortex with a diminished hand representation was crucial for the functional recovery of the affected hand, since its reversible inactivation abolished the recovered motor performance . The goal of the present report was to address the issue of possible changes in the hemisphere ipsilateral to the unilateral cervical lesion, in other words the ipsilesional hemisphere. More specifically, the following questions were addressed:
- Does a unilateral section of the CS tract at cervical level affect the hand representation in the ipsilesional motor cortex? If yes, to what extent as compared to the contralesional hemisphere? Based on the small proportion of undecussated CS axons (about 5–10%; [38–40]), one would predict a very limited, if any, impact of a unilateral cervical lesion on the ipsilesional motor cortex.
- In the context of the recovery of motor control from unilateral section of the CS tract, does the ipsilesional hemisphere play a role, in addition to the substantial contribution of the contralesional hemisphere?
Unilateral section of the CS tract at cervical level
Mapping of ipsilesional M1 hand area before and after cervical cord lesion: ICMS data
ICMS effects in the ipsilesional hemisphere. For each lesioned monkey, the total number of ICMS sites tested is given in the rightmost column, separately for the ICMS sessions pre- and post-lesion. These ICMS sites tested were then distributed in three groups, depending on whether no effect was observed ("non-microexcitable" sites) or elicited movements of the hand ("digit" sites) or movements of other body territories ("other territories"), such activation of wrist, elbow, shoulder or face muscles. Between parentheses, the number of ICMS sites in each group is given in %. For each line, the sum of the three groups is 100%.
Nb. of ICMS "digit" sites
Nb. of ICMS "other territories" sites
Nb. of "non-microexcitable" sites
Comparison of ICMS thresholds
Although both the surface of the hand representation and the number of digit sites decreased in the contralesional M1, it was observed that the ICMS thresholds in the hand area post-lesion were not significantly higher than the ICMS thresholds derived from the hand area pre-lesion . This analysis demonstrated that, at least for the contralesional hemisphere, the hand area though decreased in size as a result of the unilateral cervical lesion did not change with respect to its excitability to address the motoneurons of hand muscles, as well as the muscles of other body territories (face, wrist, elbow, shoulder, trunk). The question here is whether the ICMS thresholds were also kept unchanged pre- versus post-lesion in the ipsilesional hemisphere? To address this question, we compared the ICMS thresholds in the ipsilesional hemisphere required to elicit movements from stimulation at the same stereotaxic points before and after the lesion, in the latter case when the manual dexterity score reached the plateau. In contrast to Figure 3 where only the best ICMS site along each electrode penetration was represented, all ICMS sites of stimulation were considered. In the three monkeys, there was no systematic and statistically significant difference between the ICMS thresholds obtained in the ipsilesional hemisphere, before and after the unilateral cervical cord lesion, neither for the hand nor for other body territories (wrist, elbow, shoulder and face).
Variability of the ICMS mapping method
Two other electrode penetrations were taken from the wrist representation (tracks 2 and 4) and repeated at two time points, separated by an interval of 21 and 140 days, respectively (Fig. 5). In both tracks, the first penetration yielded several ICMS sites at which the elbow ("E") articulation was activated, replaced in the second penetrations by wrist movements in most cases. However, the lowest efficient current intensity corresponded to a wrist ("W") movement in the two tracks, both during the first and the second penetrations (Fig. 5). Again, as for digits territories, the threshold obtained for these two wrist territories remained similar at the two time points tested. Along the same line, in the electrode track located in the face representation (track 1), at the two time points tested (12 days apart), ICMS at threshold elicited movements of face muscles (Fig. 5). In summary, from the six electrode tracks repeated at two time points (Fig. 5), one can conclude that, in spite of some variability at some ICMS sites, the territory assigned to each track as defined by the effect observed at threshold did not change, even when the time interval was as long as 140 days. These observations support the notion that the size reduction of the hand area observed here in Figures 3 and 4 cannot be explained by the intrinsic variability of the ICMS method and thus are indeed related to the cervical lesion. Along the same line, a few individual electrode penetrations repeated twice before the cervical lesion in Mks1-3 also showed reproducibility of motor map body territories assessed by ICMS .
Does the ipsilesional (reduced in size) hand area contribute to the post-lesion recovery of the affected hand?
Does the size reduction of the ipsilesional M1 hand area lead to a post-lesion motor deficit of the hand contralateral to the cervical hemi-section?
To further test the motor skill of the forelimbs of Mks 1–3, the three lesioned animals also performed the so-called "reach and grasp drawer" task (see methods). In addition to precision grip skill, the drawer task tests the ability of the monkey to develop force with one or the other forelimb. The analysis of these data demonstrated that there was a deficit (time intervals and their variability were increased) for the ipsilesional arm but not for the contralesional forelimb (not shown). In line with the "Brinkman board" task, the drawer task did not show any deficit of the contralesional hand in relation to the size reduction of the ipsilesional M1 hand area.
The present results demonstrate a quite surprising and totally unexpected substantial reduction of the ipsilesional hand representation in M1, as assessed by ICMS, after unilateral section of the CS tract at cervical level. This observation appears robust since it was present in all of the three monkeys examined here. Indeed, the CS undecussated projection originating from the ipsilesional M1 and affected by the unilateral cervical lesion represents only 5–10% of the whole CS tract [38, 40, 41]. Moreover, the ICMS effects were assessed here only for the contralesional hand (Fig. 2) and thus the unilateral cervical lesion would impact only on the undecussated CS axons that cross the midline at cervical level, representing themselves only a small fraction (about 1/6) of the population of CS undecussated axons (as assessed elegantly by multiple tracing studies in monkeys subjected to cervical hemi-section at C3 level; ). In other words, based on these numbers (1/6 of 5–10% of CS axons), one would expect that the unilateral CS tract section would impact only marginally on the ICMS map in the ipsilesional hemisphere. In sharp contrast, the present data show a substantial reduction of the hand area projected on the surface of the ipsilesional hemisphere, amounting to 52%, 77% and 43% in Mk1, Mk2 and Mk3, respectively (Fig. 3), not far from the area reductions observed in the contralesional hemisphere (see ), amounting to 67%, 89% and 100% in Mk1, Mk2 and Mk3, respectively. In other words, the reduction of the hand area observed in the ipsilesional hemisphere was thus at least 50 times larger than expected, based on the very small contingent represented by the undecussated CS axons crossing the midline at cervical level.
It is important to stress that the reduction of the M1 hand area in the ipsilesional hemisphere as a result of unilateral cervical lesion in monkeys is an observation made at a specific time point, namely a few months post-lesion after the (incomplete) recovery of the affected hand had reached a plateau. The precise time course of such a reduction of the ipsilesional hand area during the few weeks post-lesion is unknown (in the absence of daily mapping) and one cannot exclude that the ipsilesional hand area was different than it appears after the recovery. Along this line, dynamic bi-hemispheric re-organization of motor networks during the recovery from hemi-paresis caused by corticospinal tract infarction has been observed . Indeed, this study showed that the early recovery of the paretic hand was correlated to a predominant activity on the intact hemisphere but, in later phases of the recovery, the activity in the lesioned hemisphere increased. One cannot exclude such progressive changes of inter-hemispheric balance between the hand areas in the two M1 in our monkeys during the recovery period, leading to the final, fairly balanced size of hand areas between the two hemispheres after recovery reached its maximum.
Comparison with previous work
The present observation of substantial plastic changes of somatotopic maps as a result of a peripheral lesion (at the level of spinal cord, or peripheral nerve lesion or amputation) is in line with an abundant literature on this topic. However, most previous studies in the monkey addressed this issue in the contralesional hemisphere with respect to the spinal cord lesion, either in the somatosensory cortex [29, 30, 39] or in the motor cortex . In human subjects too, although it is relatively rare to have a cervical cord lesion restricted to one side, most studies aimed at assessing the cortical motor re-organization after spinal cord lesion were focused on the contralesional hemisphere [36, 37]. In spinal cord injured patients, the cortical motor map changes consisted mainly of a displacement of the centre of gravity of cortical activity when using the paretic hand after partial recovery, towards a more posterior region , which was interpreted as a possible role played by the somatosensory cortex in recovery. In the monkey, as a result of unilateral cervical lesion, in the contralesional hemisphere  and in the ipsilesional one (present study), the ICMS data showed a reduction of the hand representation, but no evidence for a posterior shift of the hand representation was found. However, this discrepancy may also be explained by the difficulty to compare directly motor maps based on ICMS in the monkey and on cortical territories activated when performing movements in human. The present observation of a considerable re-organization of the motor map after unilateral cervical cord lesion in the ipsilesional hemisphere is, to our knowledge, an original observation in monkeys. Our finding can only be, to some extent, compared to previous observations in human subjects of functional reorganization in the ipsilesional hemisphere with respect to a cord damage due to lower limb amputation .
Interpretation of the motor map changes in the ipsilesional hemisphere after unilateral cervical lesion
How to explain then a plastic change in M1 in the ipsilesional hemisphere, nearly as large as that in the contralesional hemisphere as a result of unilateral section of the CS tract? The reduction of the hand area in the contralesional hemisphere was correlated with anatomical changes such as a shrinkage of the soma of layer V pyramidal neurons, involving the 90% of the axotomized CS neurons . In the ipsilesional hemisphere, we did not observe such shrinkage, as compared with intact animals , although this might have been difficult to detect since the unilateral cervical section would affect at most only 5–10% of the CS neurons, giving rise to the undecussated CS axons. In any case, the considerable plastic functional change observed for the hand representation in the ipsilesional hemisphere is not correlated with a major anatomical change (as far as the CS neurons are concerned), in contrast to the contralesional hemisphere. Consequently, the plastic change of motor map in the ipsilesional hemisphere most likely does not result from a direct impact of the axotomized CS tract. One may thus speculate that the size reduction of the hand area ipsilesionally is the result of more indirect (secondary) influences of the lesion. The present data support the notion that a reduction of the hand area in the contralesional hemisphere (which is expected) is accompanied by a nearly comparable reduction in the ipsilesional hemisphere. Although the M1 hand area is quantitatively less connected transcallosally than other body territories in M1 or other motor cortical areas such as SMA [44, 45], one may still consider the possibility that the reduction of hand area in the contralesional hemisphere provokes a "secondary" plastic change in the ipsilesional hemisphere, via the callosal projection. The process of secondary change may also occur more indirectly via non-primary motor areas (premotor cortex, SMA), which are more densely connected via the corpus callosum, since lesions of the primary motor cortex induce modifications in the premotor cortex, for instance an extension of the hand area in the ventral premotor cortex . Such "secondary" plastic change in the ipsilesional hemisphere may appear reminiscent to some extent of the transneuronal change observed in the brainstem and thalamus in adult monkeys subjected to long term dorsal rhizotomies . However, a common mechanism is unlikely because, in the case of the rhizotomy there was an anterograde plastic change induced by a lesion, whereas here after cervical lesion the impact on the CS neurons is retrograde. Moreover, a transneuronal degeneration mechanism can be excluded because most axotomized CS neurons survived to the cervical lesion, although they shrank .
The extent of the ipsilesional reduction, nearly as large as in the contralesional hemisphere, suggests that such secondary adaptive plastic change may come about as a consequence of the re-balancing of activity in the two hand areas. A roughly balanced hand area in both hemispheres is perhaps more appropriate in the context of bimanual movements as well as in the context of the functional recovery of the ipsilesional hand. Indeed, after unilateral cervical lesion, the recovery of the ipsilesional hand strongly depends on the contralesional hemisphere  and not on the ipsilesional one (Fig. 6). If the hand area in the ipsilesional hemisphere had kept its original size after lesion, then there would be a bias in favor of the intact hand, which may be detrimental for mechanisms of recovery of the affected hand. Possibly, recovery may be more efficient if the cortical area responsible for it is not too much reduced in size as compared to its counterpart in the intact hemisphere. However, the hand area in the ipsilesional hemisphere should not be reduced too much either, because this may affect the performance of the contralesional hand. In the present study, as a result of unilateral section of the CS tract at cervical level, the reduction in size in the ipsilesional hemisphere did not affect the performance of the intact hand, at least as assessed by the modified Brinkman board test (Fig. 7) or the "drawer" task. One cannot exclude that a reduction of performance may appear for more challenging tasks, involving more complex synergies of the fingers. Along this line, one may speculate that the re-sizing in the ipsilesional hemisphere should be adjusted in order to reach an ideal compromise, favoring enough the recovery of the affected hand but preserving, as much as possible, the performance of the non-affected hand. The reduction of the hand area in the ipsilesional motor cortex may also be interpreted, at least in part, by postural adjustments as well as a facilitation of those movements not affected by the lesion, such as proximal movements and, but to a lesser extent, the wrist, taking place in the contralesional hemisphere. Such contralesional motor changes, for example comprising strategies of substitution recruited for the recovery, may secondarily induce changes in the ipsilesional hemisphere as well. Postural adjustments may also include the side of the body opposite to the unilateral cervical lesion, resulting in an increased engagement of more proximal muscles at the level of the wrist, elbow and shoulder in the ipsilesional hemisphere, at the expense of the hand representation.
Plastic changes of motor maps resulting from a cortical lesion have been shown to be dependent on the level of rehabilitative training [47, 48]. It remains to be determined whether this would also be the case here in the contralesional hemisphere after cervical cord lesion and, if so, whether the same dependence on training would also be present in the ipsilesional hemisphere. In the present study, the monkeys did not undergo a particular and systematic rehabilitative training program, except for the standard behavioral tests they performed every day (essentially the modified Brinkman board test) to assess manual dexterity.
In the contralesional hemisphere, we demonstrated that rapid plastic changes of the hand motor map took place within the first few days post-lesion. During the 2 weeks after the unilateral cervical lesion, no ICMS digit sites were found . Starting about 3 weeks after the lesion, ICMS digit sites progressively re-appeared, to form the stable, reduced hand area observed several months later. Unfortunately, such a repetitive ICMS investigation was not conducted in the ipsilesional hemisphere (because no change in the ipsilesional hemisphere was expected at the time of the experiments). Such a protocol is recommended for future experiments.
Relationship between cortical plasticity and a possible role played for post-lesional recovery
Regarding the mechanisms of the incomplete recovery of the hand affected by the cervical lesion, the present study confirms the notion previously put forward  that only the contralesional hemisphere contributes to the recovered performance of the manual dexterity, as assessed by the precision grip task (Brinkman board). Indeed, reversible inactivation of the ipsilesional hemisphere did not modify the recovered manual dexterity score of the affected hand (Fig. 6). Nevertheless, we observed a change of the motor map in the ipsilesional hemisphere. It can thus be concluded that the presence of plastic changes in a certain brain region after a lesion does not necessarily mean that this region contributes significantly to the recovery. In other words, in the debate about whether the intact hemisphere plays a role in the recovery following a unilateral cortical lesion in patients (see e.g. [21–23]), a change of motor map area in the intact hemisphere as compared to normal human subjects should thus not be systematically interpreted as a contribution of the intact hemisphere to the recovery. Clearly, the strategy of reversible inactivation applicable to monkeys, as illustrated in the present study (Fig. 6), remains a better proof than just the observation of motor map changes for the actual involvement of a given brain region in mechanisms of recovery. Another conclusion of the present study is that there is no straightforward relationship between the size of the hand area in M1 (in the ipsilesional hemisphere) and the manual dexterity of the hand controlled mainly by this hemisphere. Indeed, the manual dexterity score was the same pre-lesion with a large hand area and post-lesion with a reduced hand area (Fig. 7). Furthermore, regarding the generation of force, the drawer task did not show a difference of performance pre- and post-lesion. This conclusion, valid for the motor tests used in the present study (Brinkman board and drawer tasks), may not be true for other types, of most likely more complex finger movements, as the activity in the ipsilateral motor cortex is related to the complexity of unimanual hand movements .
As a result of unilateral section of the CS tract at cervical level, the hand representation in the contralesional motor cortex was as expected dramatically affected . The present study demonstrates that a substantial post-lesional reduction of the hand representation also took place in the ipsilesional hemisphere, an original observation in the monkey. The considerable extent of the ipsilesional hand representation reduction cannot be explained by a direct effect of the lesion. Indeed, only a small number of transected CS axons originate from the ipsilesional hemisphere and could have contributed to the control of the hand opposite to the lesion by recrossing the midline below the lesion. We therefore propose that the paradoxical reduction of hand representation in the ipsilesional hemisphere is secondary to the changes taking place in the contralesional hemisphere, possibly corresponding to re-adjustments re-establishing a balance between the two hemispheres.
Overview of the experiments
The surgical procedures (anesthesia, physiological monitoring of the animal, implantation of chronic recording chamber above M1, post-operative care) were described in detail in previous reports from this laboratory [4, 5, 25, 31, 41, 49]. The experiments were conducted in three young adult male (3–4 years old) Rhesus monkeys (Macaca mulatta), Mk1, Mk2 and Mk3, weighing around 4 kg, and subjected to a unilateral section of the CS tract at cervical level C7/C8. Control experiments to test the reproducibility of the intracortical microstimulation technique were conducted in a fourth, intact monkey (Mk4; Macaca fascicularis, weighing about 3 Kg). Mk1 and Mk2 are the same two animals included in the description of motor maps changes taking place in the contralesional hemisphere  and in the anatomical modifications in M1 resulting from the cervical lesion . Mk3 underwent a comparable unilateral cervical lesion as Mk1 and Mk2, but was in addition treated during 4 weeks post-lesion with an antibody aimed at neutralizing the neurite growth inhibitor Nogo (see e.g. [50, 51]). The antibody was delivered from an osmotic pump, placed in the back of the animal, using a small silastic tube positioned intrathecally 3–5 mm above the cervical lesion. The effect of the anti-Nogo treatment in Mk3 will be reported elsewhere. Mk1 and Mk2 were also implanted during 4 weeks with an osmotic pump, but delivering a control antibody. Surgical procedures and animal care were conducted in accordance with the Guide for the Care and Use of Laboratory Animals (ISBN 0-309-05377-3; 1996) and approved by local (Swiss) veterinary authorities.
Assessment of manual dexterity
The manual dexterity of each hand was assessed in Mk1, Mk2 and Mk3 using our modified "Brinkman board" task, as described in detail earlier [25, 31, 52], testing the ability to grasp a food pellet using the opposition of the thumb and the index finger (precision grip). The "Brinkman board" is a Perspex board (10 cm × 20 cm) with 50 randomly distributed holes (15 mm long, 8 mm wide and 6 mm deep) containing each a pellet; 25 holes were oriented horizontally and 25 vertically. The task was performed daily (lasting for 15 to 20 minutes) for several months before and several months after the spinal cord lesion. All sessions were recorded on a video tape and one to two weekly sessions were analyzed quantitatively. An attempt was considered as successful when the monkey grasped a pellet and transported it to the mouth. The manual dexterity was quantitatively measured as the number of slots successfully retrieved within 45 seconds. In addition to the precision grip tested with the "Brinkman board" task, Mks 1–3 were also examined using the so-called "reach and grasp drawer task" [4–6, 53–55]. Using one forelimb (unimanual "drawer" task), the monkey had to grasp the knob of a drawer, generate enough force to pull the drawer and, finally, grasp a reward placed inside a well dug in the drawer. By means of different sensors, it was possible to measure several time intervals, separating different epochs of the task. The left and the right forelimbs were tested separately, in series of 20 trials for each forelimb (once a week).
Intracortical microstimulation experiments
The somatotopic organization in and around the hand area in M1 in both hemispheres was established based on daily ICMS sessions, as recently reported , using standard parameters of stimulation: 35 ms duration trains of 12 electric monophasic pulses (0.2 ms) presented once every 2 seconds, through a tungsten microelectrode (FHC, Maine, USA) with an impedance of 0.1 – 0.6 MOhms and a tip of about 20–30 μm. Electrode penetrations were oriented nearly perpendicular to the cortical surface, and ICMS was applied at 1 mm steps along the entire track, starting 2 mm below the dura and down to a depth of usually 8–10 mm, sometimes even deeper when the penetration went all the way down to the rostral bank of the central sulcus. On surface ICMS maps (see Figs. 3 and 4), each electrode penetration was represented by a single point corresponding to its position of entry in the brain. ICMS investigation was focused on the hand area with determination of the body territories represented a few mm around the representation of the fingers. The term "hand area" thus refers to the ensemble of ICMS sites in the motor cortex eliciting movements of the fingers observed on the contralateral hand.
The intact monkey Mk4 was included in the present study with the specific aim of assessing the variability of the ICMS method. The hand area of Mk4 was extensively mapped using the ICMS technique as described above. In the left hemisphere, six electrode penetrations selected among different body territories ("face", "wrist" and "digits") were repeated at two time points separated by a time interval ranging between 12 and 140 days.
Reversible inactivation experiments
Mks1-3 were subjected to a unilateral section of the CS tract at C7/C8 level, as described in detail recently [31, 41]. Three to five months post-lesion, a time at which the incomplete recovery process had reached a plateau, sessions of reversible inactivation of M1 in either hemisphere using muscimol were conducted, as previously described in detail [25, 31, 54]. Two to four ICMS penetration sites were chosen in the pre-lesion hand area of M1 in one or the other hemisphere (see syringes in Fig. 3) and the GABA-agonist muscimol (1 μg in 1 μl saline) was infused at two depths along each penetration (separated from each other by 2–3 mm). The infusion of muscimol was performed at ICMS sites at which finger movements were elicited at low threshold and were separated from each other in order to cover the entire hand area, based on previous experiments [25, 54]. In Mk1, three and five months post-lesion, two muscimol inactivation sessions were conducted on the left (ipsilesional) hemisphere, 7 weeks apart from each other. In each inactivation session, the total volume of muscimol injected was 18 μl, along three penetrations (Fig. 3, top left panel). In Mk2, only one muscimol inactivation session was conducted on the ipsilesional hemisphere five months post-lesion, in which a total volume of 12 μl of muscimol was infused along two penetrations (Fig. 3, middle left panel). Finally, in Mk3, one reversible inactivation session took place 4 months after the lesion, in which a total volume of 24 μl of muscimol was infused along four penetrations (Fig. 3, bottom left panel). For comparison, in the three monkeys, inactivation sessions were also conducted for the contralesional hemisphere in which muscimol infusion sites were also selected based on ICMS data (not shown; see however  for Mk1 and Mk2). The efficacy of such reversible inactivation protocol has been demonstrated previously, together with control experiments in which only saline was injected .
List of abbreviations
cingulate motor area
primary motor cortex
supplementary motor area
The authors wish to thank the technical assistance of Véronique Moret, Françoise Tinguely and Christine Roulin (histology and behavioral evaluations), Josef Corpataux, Bernard Bapst and Bernard Morandi (animal house keeping), André Gaillard (mechanics), Bernard Aebischer (electronics), Laurent Monney (informatics). Thanks are due to Dr. C. Brown for valuable comments on the manuscript.
Grant Sponsors: Swiss National Science Foundation, grants No 31-43422.95, 4038-43918, 31-61857.00 (EMR); Novartis Foundation; The National Centre of Competence in Research (NCCR) on "Neural plasticity and repair".
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